This invention relates to spectrum spreading which is preferable for use in a satellite communication network including earth stations equipped with small diameter antennas as in a VSAT satellite communication network. More particularly, this invention relates to a spectrum spreading method, a spectrum spreading device, and a spectrum spreading transmitter in each of which use is made of spectrum spreading of the type described.
Spectrum spreading of this type is already known. For example, "satellite communications system and apparatus" are disclosed in U.S. Pat. No. 4,455,651 issued to Paul Baran and assigned to Equatorial Communications Company, California, the United States of America. This satellite communication network of Baran provides a low cost satellite communication network. For this purpose, a small diameter antenna is used in each earth station for transmission and reception of radio signals. In order to trade off deterioration inevitably caused by the small diameter antenna to communication quality, a spectrum spreading technique is used on producing a transmission signal. In the earth station of the Baran satellite communication network, transmission data are spectrum spread at a spreading factor, namely, a ratio of a chip rate to an input bit rate, of the order of 256 and then subjected to BPSK modulation for transmission as the transmission signal to a satellite station.
By the VSAT satellite communication network which is used in practice in the United States of America and elsewhere, in which the input bit rate is about 64 Kbps, and in which the earth station is equipped with an antenna of a diameter of 1 meter with a transmission power of about 1 watt, it is possible without the spectrum spreading technique to provide a communication channel capable of insuring a sufficiently excellent communication quality to satisfy the FCC (Federal Communications Commission) standards. With the antenna diameter reduced to about 0.5 m, the VSAT satellite communication network would satisfy the FCC standards if the spectrum spreading technique is resorted to so as to reduce a spectrum power density off an antenna axis. In this event, a band spreading factor (hereafter referred to simply as the spreading factor) of at most about five (from four to eight) is sufficient in theory because the antenna has a gain difference of 6 dB, namely, 20 log (1/0.5) dB.
Spectrum of a spectrum spread signal will now be studied in connection with this small spreading factor. It will be assumed that the spreading factor is equal to four.
It is usual in this event to use four as a code length of spreading codes for use in spectrum spreading an input bit sequence into a spectrum spread signal. In order to reduce a peak spectrum power density, important is whether or not it is possible with the spreading codes of a four-bit code length to get the spectrum spread signal with a sufficiently smooth spectrum spreading characteristic.
Attention will be directed under the circumstances to spreading codes "0000" and "1111". These spreading codes provide two emission line spectra. The spectrum spread signal can not have a smooth spectrum. Spreading codes "1010" and "0101" also provide emission line spectra. With these spreading codes, an unevenness results in the spectrum spread signal. With spreading codes "1100" and "0011", emission line spectra again appear in the spectrum of the spectrum spread signal. When the spreading factor is four, there are ten other spreading codes. All of these spreading codes do not give a sufficiently smooth spectrum to the spectrum spread signal.
In the manner which will later be discussed more in detail, it has been necessary for smoothening the spectrum of the spectrum spread signal to use the spreading codes with their code length selected between 100 and 1,000. The spectrum spread signal, however, has a much widened signal bandwidth in this event. As a consequence, the transmission signal would objectionably give interference to adjacent signals when the satellite station is used in common.